The Sagittarius dwarf galaxy is our nearest neighbor. Yet it has been
discovered only recently, in 1994, being hidden by the stars and dust
in our own Galaxy, the Milky Way. It is however possible today to
better know this companion galaxy, thanks to variable stars, the RR
Lyrae, in which Sgr-dw is particularly rich. In a recent paper, Patrick
Cseresnjes, from Paris Observatory, shows for the first time that
Sgr-dw is not typical of other satellites of the Milky Way, but reveals
instead striking similarities with the Large Magellanic Cloud. He
proposes and argues for the astonishing and original scenario that both
systems might share a common progenitor.

The Sagittarius dwarf galaxy (Sgr hereafter) is a most interesting
object. Located at only 75,000 light-years from the Sun and 50,000
light-years from the Galactic Center, it is the nearest known satellite
of the Milky Way. In spite of this proximity, Sgr has been discovered
only in 1994 because it was hidden to us by foreground Galactic stars.

Sgr is now in process of being swallowed by our own Galaxy after
complete disruption caused by Galactic tides, showing that at least
part of the stellar Halo has formed from accretion of smaller
constituents. However, we still lack a clear understanding oéª-ois
galaxy because the high degree of contamination by foreground Galactic
stars and the varying extinction make it almost impossible to get a
clean sample of stars. Fortunately, Sgr contains a fair amount of RR
Lyrae stars. These variable stars have characteristic light curves
and can easily be detected and separated from Galactic stars. Indeed,
once their type is identified by their light curve, their absolute
luminosity is derived, and the measure of their apparent luminosity
gives their distance.

Using two series of photographic plates, taken at La Silla (European
Southern Observatory) and digitized by the MAMA (operated at the Centre
dÕAnalyse des Images, Observatoire de Paris), Patrick Cseresnjes and
his collaborators detected about 2000 RR Lyrae stars in Sgr spread over
50 square degrees. The spatial distribution of these stars allows to
map the northern extension of Sgr, where the Galactic stars outnumber
those of Sgr by a factor up to a thousand. Compared to other satellites
of the Milky Way, Sgr seems to be much more massive and extended.

Stellar evolution theory indicates that RR Lyraes are more than 10
Gigayears old. A catalogue of such stars offers therefore an unique
opportunity to determine the progenitor of Sgr. The most obvious
information available is the period which is very accurate and
independent of crowding and extinction, allowing robust comparisons
between different systems. Patrick Cseresnjes and his collaborators
compared the period distribution of RR Lyrae stars in Sgr with those
of all other dwarf galaxies with a known RR Lyrae population. The
similarity with the Large Magellanic Cloud (LMC) clearly stands out.
This similarity is even more striking when one considers that there
are no two other couple of distributions showing such a high
correlation. Statistical tests show that an identical parent
distribution for Sgr and the LMC cannot be ruled out, in spite of
the high resolution provided by the large size of the samples in both
systems.

The similarity between Sgr and the LMC is not restricted to RR Lyrae
stars, but has also been observed through other populations like
Carbon stars, in 1998 or Red Giant Branch stars, in 2001. These
similarities strongly suggest that both systems have similar stellar
populations. So, Sgr could be a debris pulled out of the LMC after a
collision and has been injected on its present orbit only recently.
Possible configurations are a collision between the LMC and the
Galaxy or the Small Magellanic Cloud.

This scenario, though attractive, raises many questions which need to
be addressed. When did the collision occur? What happened to the gas?
How can the present orbital planes of Sgr and the LMC seem to be
perpendicular to each other? Future numerical simulations will assess
the feasibility of this scenario.

References:

* Cseresnjes P., 2001, “RR Lyrae stars in the Sagittarius dwarf galaxy:
Period analysis”, Astronomy & Astrophysics 375 909

* Cseresnjes P., Alard C. & Guibert J., 2001, “Structure of the
Sagittarius dwarf galaxy at low Galactic latitudes”, Astronomy &
Astrophysics 357 871

* Cseresnjes P., Ph.D. Thesis, Observatoire de Paris

[NOTE: Further information and images supporting this release are
available at
http://www.obspm.fr/actual/nouvelle/feb02/cseresnjes.en.shtml]

Patrick Cseresnjes

Observatoire de Paris (GEPI) and Columbia University, New York

Tel: 33 1 40 51 20 95

Fax: 33 1 40 51 20 90

E-mail: Patrick.Cseresnjes@obspm.fr